A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). As semiconductor thin films applicable to the transistors, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
For example, disclosed is a transistor whose active layer is formed using an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of lower than 1018/cm3 (see Patent Document 1).
A transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of fluctuation in electrical characteristics, although the transistor including an oxide semiconductor can be operated at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. For example, the threshold voltage of the transistor fluctuates between before and after a bias-temperature stress test (BT test). Note that in this specification, a “threshold voltage” refers to a gate voltage which is needed to turn on a transistor. “Gate voltage” refers to a potential difference between a source electrode and a gate electrode when the potential of the source electrode is used as a reference potential.